The present invention relates to a process for drying timber in a drying tunnel through which the timber is fed stepwise and is there permeated by drying air below 80.degree. C. which flows in the longitudinal direction of the tunnel. In particular, the present invention relates to such a process in which the drying tunnel is divided into two sections separated by an intervening space and with the drying air divided into two circulating substreams. By means of the process, improved quality of the dried timber is obtained with unchanged drying time or, alternatively, a shorter drying time with unchanged quality level.
Sawed timber should be dried to a moisture content of approximately 15-22%, calculated on the dry weight of the timber, in order that the timber can be stored without biological attack in the form of mold, etc. For drying timber at sawmills, two main types of drying kiln are employed, so-called compartment kilns and progressive kilns (tunnel kilns), whereas timberyard drying has practically ceased.
In the compartment kiln, the entire quantity of timber which is to be dried is loaded into the kiln at one time and stacked in piles in known fashion. In principle, any drying schedule whatsoever can be achieved in a kiln of this type. The drying schedule is determined by the temperature and moisture content of the drying air and its speed of flow through the timber pile and their variations during the drying period. It is therefore possible in this type of kiln to employ what is, by some criterium, the optimum drying schedule. This is the principal advantage of this kiln. The disadvantages include a relatively high energy consumption and that these kilns cannot be made especially large because otherwise the drying climate would vary too much in different parts of the timber load.
In a conventional single-stage progressive kiln the piles of timber move stepwise through the tunnel while new piles are loaded at regular intervals and at the same time dried piles are taken out from the other end of the tunnel. The drying air flows along the length of the tunnel in a counter-current direction through the piles. As the drying air flows through the piles it is cooled at the same time as its moisture content rises. Once the condition of the drying air which is fed into the tunnel and its speed have been chosen the changes in the temperature and moisture content of the air (i.e., the drying schedule) can no longer be controlled but depend only on the interaction of the wood with the air flow. Thus, unlike the compartment kiln, in a single-stage progressive kiln it is not possible to achieve any optimum drying schedule. Against this, the progressive kiln has the advantage that the energy consumption is appreciably lower since the air which leaves the kiln is almost saturated and also heat recovery can readily be obtained. Further, the progressive kiln can advantageously be constructed for high capacities, 10,000-20,000 m.sup.3 /annum.
A division of the progressive kiln into two stages has been proposed and has also come into use at some sawmills. In such a two-stage kiln, the drying air is introduced into the tunnel between the stages so that part flows in a countercurrent direction in the first stage of the kiln and part in a concurrent direction in the second drying stage. Compared with the single-stage progressive kiln, this two-stage progressive kiln has advantages primarily in regard to control technology as it has some self-regulating properties.
In the choice of the drying schedule for a compartment kiln or of the condition of the inlet air for a progressive kiln, there are two main requirements which should be satisfied. On the one hand the final moisture content of the timber after the desired drying time should be that which is aimed for, and on the other hand the quality loss of the timber in drying should be as little as possible, or at least acceptable. In general, the speed of drying increases as the difference between the dry-bulb and wet-bulb temperatures of the air increases. The magnitude of the change in the quality of the timber is a more complicated function of the drying procedure, but roughly it can be said that the faster drying is carried out the greater are the quality losses. Thus, in general, it is a question of a compromise between slow drying with low throughput but good quality and fast drying with reduced quality. The compartment kiln has thereby achieved an increased importance for drying with preservation of quality since in such a kiln the drying schedule can be chosen in an optimum fashion. Drying can namely be carried out relatively fast without accentuating the quality loss.
Against the background of the aforesaid circumstances there has been a clear effort to try to construct progressive kilns with the characteristic advantages of this type but so that the disadvantage of the non-optimum drying schedule can be circumvented.
The quality loss of the timber in drying can be divided into two main components. One is that with high temperature levels and/or long drying times there is a flow of resin at knots, etc., together with a darkening of the surface of the timber. The other is the occurrence of checks in the timber. Of these two groups, check formation is, especially with thicker dimensions, clearly the more important. The cause of check formation can be explained in the following manner. In drying, the surface of the timber dries faster than the inner parts of the piece of timber because of the resistance to the movement of moisture within the material. When the fiber saturation point is reached, i.e., when the free water has been removed and only water bound to the wood substance remains, the wood starts to shrink. This means that an internal mechanical tensile stress arises in the surface of the timber. This tensile stress by shrinkage is balanced by a corresponding compressive stress in the inner parts of the timber. If the tensile stress in the surface layer exceeds the strength of the wood, a rupture takes place, i.e., surface checking occurs. Thus, it is clear that if the difference in moisture content between the surface of the timber and its inner parts (the moisture profile) is pronounced, the risk of check formation increases, i.e., in rapid drying the risk increases. The matter is complicated, however, by the fact that wood is not a purely elastic material but exhibits viscoelastic properties. This means, e.g., that if the surface of the timber is subjected for a longer time to tensile stress then creep occurs, i.e., there is a permanent extension of the surface layer. When drying has progressed so far that also the inner parts have reached the fiber saturation point, the surface has accordingly extended more than the inside and the stress pattern is then reversed so that the outside is subjected to compressive stress and the inner parts to tensile stress. Hence, during this latter phase of drying, internal checking of the timber can occur. Though these internal checks cannot be seen they are of great importance in possible subsequent working of the timber.
At elevated temperatures, the flow of resin and darkening of the surface is a problem for several wood species, especially softwoods. It is known from practice that the rate of quality change due to these phenomena usually doubles for every 10.degree. C. increase in temperature. For this reason, temperatures above about 80.degree. C. cannot be used in many cases. This fact emphasizes the need to control drying induced checking because stress relaxation through creep behavior of wood is slowed down when the temperature is low.
Even though both mechanisms of quality loss described above and the mechanisms of moisture transport have long been known at a qualitative level, the development of improved drying schedules has been almost exclusively empirical, i.e., based on direct experience concerning the final moisture content and quality which is obtained with the drying schedule which is tested. It can also be stated that the continuous measurement of the moisture content and profile of the timber during drying is admittedly possible but in practice very troublesome. On the other hand, so far, there exists no reliable method for the continuous measurement of the stress conditions in the timber or even for registration of when checks occur.
It has, however, now proved possible with the aid of physical and mathematical methods of calculation to predict in a reliable fashion on the one hand how the moisture content and moisture profile of the timber develops and varies in different drying climates, and on the other to predict on the basis of these profiles what stresses occur and thus the risk of check formation. Similarly, there are possibilities to estimate resin flow and the color change of the timber surface. Thus, the final moisture content of the timber with a given drying schedule can be calculated and also the quality loss can be predicted with such models.